Diffusion of small solutes in polymer-containing solutions.

Diffusion processes involving polymers are common in scientific and engineering separations and are a major component of biological functions. Analyses of these systems are usually based on versions of the Stokes-Einstein equation, although order of magnitude deviations have been observed. Presented here is a theoretical correction to the Stokes-Einstein equation containing a "local viscosity" function that combines diffusional hydrodynamics with Maxwell's treatment of electrical resistance in inhomogeneous regions. The resulting equation accurately predicts experimental diffusion data within tight bounds for polymer concentrations from 0 to 9 percent. It requires knowledge only of thermodynamics and of pure solvent and solution viscosities.